1. Field of the Invention
The present invention relates to a method for forming a bottle trench, and more particularly to a method for forming a bottle trench using an etching stop layer.
2. Description of the Prior Art
Generally, capacitors in presently extensively used dynamic random access memory (DRAM) include two electrode plates with an insulating material therebetween. The charge storage capacity of the capacitor depends on the insulating material thickness, the surface area of the electrode plates, and the electrical properties of the insulating material. In recent years, the semiconductor design has a trend toward decreased device size in order to obtain higher density. Therefore, the substrate area of the memory cell must be decreased in order for the integrated circuits to accommodate a large number of memory cells. Also, the electrode plate of the memory cell capacitor must have adequate surface area in order to store sufficient charge.
However, with reduction in size, the trench storage node capacitance in the DRAM decreases accordingly. Therefore, there is a need to increase the storage capacitance in order to maintain sufficient memory operation.
Presently, one popular method of increasing the storage capacitance of DRAM forms a bottle shaped capacitor. This increases the width of the trench bottom for increased surface area. FIGS. 1A to 1H are cross-sections illustrating the conventional process flow of forming a bottle trench. First, referring to FIG. 1A, a pad structure 16 including a pad nitride layer 14 and a pad oxide layer 12 is formed on a silicon substrate 10. Then, the pad structure 16 is used as an etching mask to etch the silicon substrate 10 by dry etching, forming a trench 18.
Subsequently, referring to FIG. 1B, a pad oxide layer 20, an etching stop layer 22, and a polysilicon layer 24 are successively formed in the trench 18. Then, an oxide layer 26 is formed by thermal oxidation. Then, a mask layer 28, for example, a photoresist layer, is deposited. The etching stop layer 22 can generally be a nitride layer.
Subsequently, referring to FIG. 1C, the mask layer 28 at the upper portion of the trench 18 is removed.
Subsequently, referring to FIG. 1D, the oxide layer 26 uncovered by the mask layer 28 at the upper portion of the trench 18 is removed.
Subsequently, referring to FIG. 1E, the remaining mask layer 28 is removed.
Subsequently, referring to FIG. 1F, the upper portion of the polysilicon layer 24 is nitrided to form a nitride layer 30.
Subsequently, referring to FIG. 1G, the oxide layer 26 and the polysilicon layer 24 uncovered by the nitride layer 30 at the bottom portion of the trench 18 are removed by wet etching. Next, the nitride layer 30 and the nitride layer 22 at the bottom portion of the trench 18 are removed by wet etching using phosphoric acid.
Finally, referring to FIG. 1H, the residual polysilicon layer 24 at the upper portion of the trench 18 is removed. The pad oxide layer 20 at the bottom portion of the trench 18 is removed by wet etching using the etching stop layer 22 as a mask.
However, the above conventional process uses complicated stacked sacrificial layers. Also, the process is very complicated, and the process time is long, proving detrimental to mass production.
An object of the present invention is to solve the above-mentioned problems and provide a method for forming a bottle trench. The benefit of the present invention resides in that TEOS (tetra-ethyl-ortho-silicate) is used as a sacrificial etching stop layer in the bottle trench formation. The TEOS layer can be easily removed with hydrofluoric acid solution.
To achieve the above object, the present inventive method for forming a bottle trench in a substrate having a pad structure and a trench includes the following steps. First, a first insulating layer is formed in the trench, and then a portion of the first insulating layer is removed to a certain depth of the trench. Next, a second insulating layer is formed in the trench, and portions of the second insulating layer on the pad structure and the sidewalls of the trench are removed. Next, an etching stop layer is formed in the trench, and a bottom portion of the etching stop layer is removed. Finally, the etching stop layer is used as a mask to remove the remaining second insulating layer and the first insulating layer.